Left channel tubes start popping, and then go dim with lots of noise.

Anyone experience the same problem?

it would RALLY help us if you can:
1/ tell us what model amp
2/ what types of output tubes
3/ at what volume level
4/ when did this start

the more information we get, the better chance of getting any replies to your post!

1/ Dynaco 70
2 / EL34 JJ
3 / Idle
4 / Monday night

OK, good. Is this ST70 an original model, or a new VTA ST70, if it is an original, approximately how old is it.
If it is an original unit, the tube/s could be on their way out.
Also, check that the pins in the tube sockets are tight and clean, as well as the pins on the tubes.
What is the BIAS, have you recently checked that.

mlabrash wrote:1/ Dynaco 70
2 / EL34 JJ
3 / Idle
4 / Monday night

You most likley have 2 7199 tubes and a gz34 . The gz34 is used for both channels and thus
cannot gie the observed symptoms. The 7199 can. Switch them and see if the problems
moves to the other side.

Do not switch the EL34 unless you are prepared to do a full bias adjustment!

If swapping 7199 does not identify the problem, prepare to hand it over to a technitian.

Peter,

You may not have noticed he said the EL34s go dim when this occurs. The 7199 is not likely to be able to do that! But with JJ output tubes and noises/light shows I'll bet on a bad EL34. A heater-cathode short would be pretty exciting to see and hear, and that may be what he has.

So he should swap the 7199s just to rule them out (but do it quickly, this amp is potentially in self-destruct mode!), but I suspect the power tubes or the sockets they are plugged in to.

Jim McShane wrote:Peter,

You may not have noticed he said the EL34s go dim when this occurs. The 7199 is not likely to be able to do that! But with JJ output tubes and noises/light shows I'll bet on a bad EL34. A heater-cathode short would be pretty exciting to see and hear, and that may be what he has.

So he should swap the 7199s just to rule them out (but do it quickly, this amp is potentially in self-destruct mode!), but I suspect the power tubes or the sockets they are plugged in to.

without some more info, at this stage too I would bet on a bad EL34 or bad connection/s in the sockets.

MontanaWay wrote:

Jim McShane wrote:Peter,

You may not have noticed he said the EL34s go dim when this occurs. The 7199 is not likely to be able to do that! But with JJ output tubes and noises/light shows I'll bet on a bad EL34. A heater-cathode short would be pretty exciting to see and hear, and that may be what he has.

So he should swap the 7199s just to rule them out (but do it quickly, this amp is potentially in self-destruct mode!), but I suspect the power tubes or the sockets they are plugged in to.

without some more info, at this stage too I would bet on a bad EL34 or bad connection/s in the sockets.

Agreed. Does it begin doing this immediately after it's powered up, or does it take some time for it to happen?

-Pat

way not enough info - we can guess this is a 50 year old amp, but it may be a newborn repro.
It may have a 50year old PCB, or a new VTA70 driver.   It may have any kind of driver tubes.
The key to correct answers is information

mlabrash wrote:Anyone experience the same problem?

if you want us to help you, we need to have some of the questions in this thread answered.

OK, I got the scoop from this user, it's an old Dynaco, mostly original.
Here's the info I gave him . . . .

OK, so now some real info, thanks. Two possible problems, but you did say you were measuring bias at 1.2v,
because that's suspect #1. Suspect number two is bad tube sockets on that side (very unlikely, although see my final note at the bottom of this message)
so we're back to suspect #1, bias is not correct, and too much current thru the output tubes. Check a couple things.

On an original Dynaco there is a common 15.6 ohm resistor connected to the two output tubes on each side of the amp.
That is where you would be measuring 1.2v of bias (although there is most likely a wire being run to the front octal sockets as a test point).
Make SURE that the solder connections on both tubes is good there (pins 1 & . Also check to make sure there is a 1K resistor from
the PCB outputs to pin 6 on each tube. Finally, without any tubes in the amp (none at all) measure the voltage on pin 5 of each output tube.
It should be between -35vdc to -40vdc.
Also use an ohm meter to make sure BOTH bias pots are working correctly.

Finally, make sure the pins in the tube sockets are holding the tubes tightly. Some newer tubes have slightly smaller pins than did the tubes
from 20-50 years ago, and if the tube isn't making a tight connection on EVERY single pin, then you can have this problem.